Audio Research M300 monoblock power amplifier Page 4

Martin Colloms closes

Until recently, the largest Audio Research power amplifier was the D-250, a massive, fan-cooled stereo chassis offering 250Wpc into a well-matched load. With its 16 output tubes, plus many others besides, designing and manufacturing the D250 represented an act of considerable courage on the part of ARC! Now in Mk.II form, the D-250 stands as one of the great high-power amplifiers.

Traditionally, ARC power amplifiers were stereo units, but the virtues of monoblock operation were also explored in a version of the D115 called the M100. Ultimately this proved to be considerably different in engineering design and produced a decidedly superior sound. Many aspects of audiophile circuit design were analyzed in the development of the M100, and the importance of wide-bandwidth drive, as well as good balance, was established for the output stage. The merits of larger supply storage and better regulation were also seen in this highly musical 100W amplifier, a model of classic quality.

Thus the stage was set for the design of a full-sized monoblock version of the D250—one which would reach beyond the '250 by as much as the M100 overtook the D115. FET input circuitry derived from the new technology learned in the development of the SP-11 preamp has been combined with the first commercial use of a new tube output stage circuit, resulting in a world-class high-power monoblock using hybrid circuitry, continuously rated at a huge 300Wpc.

This is a big power amplifier, though more wieldy than the D250. Monoblocks allow siting nearer to the speakers, reducing costly speaker-cable runs, and potentially improving that link, while the longer runs of pre-power cable interlink are easier to use. This may also offer a performance advantage compared with short preamp and long speaker cables. JGH excepted, if you are feeling fit, a M300 may be moved by a single person if some care is taken; this is not true of the D250, except where professional weightlifters are concerned!

The M300 is, in fact, the first in a series of new or revised ARC power amplifiers which will exploit the high-efficiency output stage. As a result, it is expected that the new transformer secondary configuration introduced in the M300 (and long overdue) will appear on later smaller models. Traditionally, ARC amplifiers have used a balanced secondary winding for load matching, but true balance was only theoretically possible for a 16 ohm load (this impedance a relic from the early days of tube and loudspeaker design). All other taps, 8 and 4 ohm, were effectively unbalanced.

The M300 has a power rating which is no longer defined for 16 ohm loads. Instead it is specified for three impedance-matching conditions—8, 4, and 2 ohms—systematically loaded onto the pairs of balanced secondary taps provided. It is also possible to use just half of the secondary in unbalanced mode, this providing two 2 ohm and one 1 ohm matched taps, though at the expense of a loss of power of about 1dB.

Built in classic ARC style, the M300 looks rather like an oversized D115 or M100, clearly differentiated by the two horizontal blocks of four output tubes each fitted in the rear "cage." Handles are provided front and rear to aid shifting, while the front-panel bias adjuster and LED array of the M100 is absent, this feature being no longer required. In addition to power "on," the other lever switches are "operate" or "standby," the latter a cool condition but not one in which the amplifier can be auditioned, and finally two fan speeds for internal cooling.

Standby reduces the standing current consumption from 450 to 130W, by resting the output valves electronically while maintaining full cathode temperatures and all the normal internal operating conditions. Thus the amplifier may be rested in standby and will hit full performance 10 minutes or so after switching to "operate." A brand-new amplifier's sound continues to improve over the first few days of use; subsequent full performance will not be attained until 20-40 minutes after a "cold" start. ARC does not recommend permanent standby operation, however, merely its use as an advance preconditioning prior to a listening session, or when natural pauses occur. Since a pair of operating M300s push nearly a kilowatt of energy into the room, more when running hard, standby also helps to keep the room cooler. This is not an idle comment, since I have lived with hot amplifiers for some years now and have experienced stifling room conditions during hot weather.

In my listening room, which is not airconditioned but is double-glazed to reduce ambient noise, an external temperature of 75 degrees F will mean 90 degrees F indoors after a day's listening using Krell KMA-100s or the like; this should be borne in mind. In demanding low noise levels I take a critical view of cooling fans in amplifiers, frequently finding them too loud for comfort, except when tucked away in very large listening rooms. For example, in the Krells the fans were specially selected and I carried out further modifications to quiet them even more. I sometimes listen at low levels late in the night and demand near-silent backgrounds.